A civilization might select optical wavelengths for communication. Detecting such signals may be difficult. Prior systems assume that such signals have the following characteristics:                number of particles received must significantly exceed the background,        signal must exhibit some non-natural property,        energy per quantum should be minimized, other things being equal,        negligible absorption by interstellar medium (and atmosphere),        negligible deflection by galactic fields,        should be easy to generate, launch, and capture, and        velocity should be as high as possible.        
Prior systems have used variants of photomultiplier tubes or atmospheric Cherenkov radiation as detectors. These techniques, however, are not cost effective enough to cover all the sky, all the time.